After upgrading from Leopard to Snow Leopard, I came across a popup message that mentions installing Rosetta. First time I have heard about it with regards to Mac OS. So what is Rosetta? It is actually an optional installation in the Snow Leopard Mac OS installer which occupies quite a small file size (around 2MB) and is actually used to run PPC applications in an Intel based laptop or desktop.
A common case is Microsoft Office 2008. If you already have it installed prior to upgrading to Snow Leopard, then you do not have problems. You can freely run Microsoft Office applications anytime. But, if you wish to re-install it again, Rosetta would be needed as the installer is a PowerPC application.
Having Rosetta in your system does not seem to degrade system performance. It only runs when it is needed so no worries about it running in the background all the time.